The present invention relates to a grinding machine, and more particularly to a gear grinding machine for grinding a workpiece or gear with a short period of time by adjusting the speed of relative movement of the workpiece and a grinding wheel when the workpiece is ground by the grinding wheel as it rotates.
There have heretofore been used in the art grinding machines for grinding workpieces such as gears with a grinding wheel having helical teeth on an outer peripheral surface thereof which are held in mesh with gear teeth. Since it is necessary to grind the workpiece fully across its width, the workpiece and the grinding wheel are moved relatively to each other in a process known as traversing. FIG. 1 of the accompanying drawings illustrates such a traversing process. The workpiece, designated 2, is rotated about its own axis by a rotating shaft 4, while the grinding wheel, designated 6, is traversed axially of the workpiece 2 from a position A through a position B to a position C to grind the workpiece 2 across the entire width thereof. The workpiece 2 could also be ground across the entire width by moving the workpiece 2 while holding the grinding wheel 6 at rest.
When the grinding wheel 6, that is, the center thereof, is in the position B with respect to the workpiece 2 during the traversing process, the area of contact between the grinding wheel 6 and the workpiece 2 is maximum and so is the load imposed on a motor which rotates the grinding wheel 2. If the workpiece 2 has an eccentric center, varying grinding allowances, or strains caused by a heat treatment before it is ground, then the workpiece itself is not homogeneous, and the load imposed by the workpiece on the grinding wheel varies as the workpiece makes one rotation.
Therefore, the grinding wheel motor is subjected to varying loads due to varous irregularities of the workpiece and the traversing movement of the grinding wheel. At any rate, when the load on the grinding wheel motor exceeds the ability of the grinding machine to control the grinding wheel and the workpiece to operate in synchronism, the accuracy with which the workpiece is ground is appreciably reduced or the grinding wheel is damaged.
To avoid the foregoing difficulties, prior gear grinding machines are designed such that a maximum load on the grinding wheel motor during traversing movement of the grinding wheel is assumed beforehand, grinding conditions such as a traversing speed, a range of grinding allowances, and the like which match the maximum load are established, and the grinding wheel is traversed according to the established grinding conditions. FIGS. 2(A) and 2(B) of the accompanying drawings shows the relationships of the traverse table displacement and the load to time when the grinding wheel is traversed at a constant speed according to the above process while grinding the workpiece. As shown in FIG. 2(A), the grinding wheel is brought into mesh with the workpiece while positioning the grinding wheel in advance to ensure a small depth of cut to allow the grinding wheel to cut reliably into the workpiece (see a in FIG. 2(A)). As the cutting movement is continued, the load on the grinding wheel is gradually increased (see a in Fig. 2(B)). When the grinding wheel starts being traversed across the workpiece (b in FIG. 2(A)), the load on the grinding wheel motor immediately reaches a peak (b in Fig. 2(B)). On continued traversing movement of the grinding wheel (c in FIG. 2(A)), the load on the grinding wheel motor experiences several peaks (c in FIG. 2(B)) and is then reduced when the traversing stroke is finished (d in FIG. 2(A)). When the grinding wheel starts to be traversed in an opposite direction (e in FIG. 2(A)), the motor load is increased again to a peak (f in FIG. 2(B)) in a final stage of the traversing stroke (f in FIG. 2(A)).
With the conventional arrangement, therefore, if one cycle of traversing movement of the grinding wheel is to be completed within a given period of time irrespectively of variations in the load applied to the grinding wheel motor, the grinding allowance to be ground by the traversing of the grinding wheel is reduced, and any remaining grinding allowance is carried over to a next traversing cycle. As a consequence, the overall grinding time is prolonged. For any workpiece which will impose a load in excess of the predetermined maximum load on the grinding wheel motor, no desired grinding accuracy would be ensured. If such a workpiece were ground, the grinding wheel would tend to be damaged.
With the prior drawbacks in view, it is an object of the present invention to provide a grinding machine which can grind a workpiece in a short period of time and with a sufficient grinding accuracy, and is prevented from being damaged in the grinding operation.
According to the present invention, there is provided a grinding machine comprising a first rotational drive source for rotating a grinding tool, a second rotational drive source for rotating a workpiece to be ground by the grinding tool, means for displacing the grinding tool and the workpiece with respect to each other, and a control unit for controlling the displacement effected by the displacing means dependent on a variation in a load imposed on the first rotational drive source.
The displacing means comprises a traverse table and a traverse motor for moving the traverse table.
The traverse table supports thereon the workpiece and the second rotational drive means, the workpiece being displaceable with respect to the grinding tool by the traverse table in response to energization of the traverse motor.
The displacing means also includes a ball screw coupled to a rotatable drive shaft of the traverse motor for displacing the traverse table in response to rotation of the ball screw about its own axis.
The control unit comprises a control circuit composed of a power detector connected to the first rotational drive source for detecting the load on the first rotational drive source, a comparator connected to an output terminal of the power detector for comparing an output load signal from the power detector and a normal load setting signal, and an adder for adding an output signal from the comparator and a command signal for a feed speed of the traverse table.
The control circuit has a proportional-plus-derivative control circuit connected to an output terminal of the comparator, an analog switch connected to an output terminal of the proportional-plus-derivative control circuit, and a zero-crossing detector connected to the output terminal of the comparator for selectively opening and closing the analog switch in response to the output signal from the comparator to control the application of an output signal from the proportional-plus-derivative control circuit to the adder.
The comparator issues an output signal to the proportional-plus-derivative control circuit by subtracting the output load signal of the power detector from the normal load setting signal.
The adder issues a control signal to the traverse motor by substracting the output signal of the proportional-plus-derivative control circuit from the command signal for the feed speed of the traverse table.
As an alternative, the control unit comprises a power detector connected to the first rotational drive source for detecting the load on the first rotational drive source, a comparator connected to an output terminal of the power detector for comparing an output load signal from the power detector and a normal load setting signal, an amplifier connected to an output terminal of the comparator, a relay connected to an output terminal of the comparator, and a switch selectively openable and closable under the control of the relay for controlling the application of a control signal to the traverse motor.
The switch comprises a normally closed switch which can be opened in response to energization of the relay.
The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.